The invention relates to an axle assembly with level control and pneumatic suspension for a vehicle, in particular a non-trackbound vehicle such as a large-capacity passenger car, having controllable pneumatic-suspension assemblies, for example pneumatic-suspension bellows, which are separate for each vehicle side and which can be connected separately to a compressor and/or to an outlet to the atmosphere via a control valve arrangement, and having sensors, which are separate for each vehicle side, for sensing the level of a vehicle body and/or for sensing a distance between a vehicle body-end part and a wheel carrier of the axle assembly.
In contemporary vehicles with pneumatic suspension, a compressed air accumulator is normally provided, which accumulator is continuously kept at a charging pressure above the operating pressure in the pneumatic-suspension assemblies by means of a compressor. In the case of a raising of the level of the vehicle body on both sides, which may possibly become necessary, the pneumatic-suspension assemblies of both vehicle sides are firstly connected to the pressure accumulator. As soon as the desired level is then at least approximately reached on one vehicle side, the connection between the pneumatic-suspension assembly of this vehicle side and the pressure accumulator is interrupted, while the pneumatic-suspension assembly on the other vehicle side continues to be connected to the pressure accumulator until the desired level is also at least approximately reached there. In this way, a vehicle can, even in the case of unequal loading of the vehicle sides, be placed with its transverse axis parallel to the carriageway or the underlying surface. The differences in loading between the vehicle sides are compensated by appropriate pressure differences between the pneumatic-suspension assemblies.
In the case of a lowering of the level on both sides, which may possibly become necessary, the pneumatic-suspension assemblies of both vehicle sides can firstly be simultaneously vented by means of outlet valves which are assigned separately to each pneumatic-suspension assembly, until the desired level is at least approximately reached on one side. Subsequently, only the pneumatic-suspension assembly on the vehicle side with excessively high pressure is then vented further.
DE 36 44 942 C2 discloses a hydropneumatic suspension system for vehicles in which parts of the vehicle can be lowered or raised even outside the control range of a level control system. So that only a comparatively small degree of expenditure on control is necessary in the case of such extreme lifting or lowering movements of the vehicle body, this publication provides for simultaneous operation of the supporting assemblies which are involved in the lifting or lowering movement to be achieved by virtue of the fact that these supporting assemblies are each briefly individually activated in succession in a predefined sequence, i.e. additional hydraulic medium is briefly fed to the hydraulic cylinder of the respective hydropneumatic supporting assembly when the vehicle of the vehicle parts is to be raised. If a lowering movement has to be carried out, hydraulic medium is briefly removed from the hydraulic cylinders of the supporting assemblies in succession. Here, the fact that the compressibility of hydraulic media is extraordinarily low is exploited. Therefore, if hydraulic medium is briefly fed to the hydraulic cylinders of the hydropneumatic supporting assemblies in succession or hydraulic medium is briefly drawn off from the hydraulic cylinders in succession, it is possible, by means of a simple measurement of time or quantity, to ensure that, averaged over time, all the hydraulic cylinders carry out the same vertical adjustment and a tilting movement of the vehicle in the course of the lifting or lowering movement is avoided.
The object of the invention is to reduce as far as possible the expenditure on open-loop or closed-loop control in a pneumatic-suspension system and at the same time permit a high level of comfort.
For this purpose, there is provision according to the invention that a change in level on both sides takes place incrementally and with continuous alternation between the vehicle sides in that, when the level is being raised on both sides, a closed-loop or open-loop control unit which is assigned to the control valve arrangement directly connects the pneumatic-suspension assembly of the one vehicle side and the pneumatic-suspension assembly of the other vehicle side to the compressor in an alternating fashion in order to carry out successive small or limited lifting movements, and, when the level is being lowered on both sides, it connects them to the atmosphere in an alternating fashion in order to carry out successive small or limited lowering movements, in which case, during the respective lifting or lowering movement at the respectively active pneumatic-suspension assembly, a level is set which is higher or lower, by a specific amount, than the level at the corresponding pneumatic-suspension assembly on the respective other vehicle side.
The invention is based on the general idea of normally avoiding a simultaneous adjustment of the pneumatic-suspension assemblies on both vehicle sides of an axle and of carrying out changes in level in each case incrementally and with continuous alternation between the vehicle sides. As a result, when there are large differences in loading between the vehicle sides and corresponding differences between the supporting forces and/or the pneumatic forces of the suspension assemblies, a load equalization or pressure equalization between the assemblies and a pronounced rolling movement of the vehicle body relative to the longitudinal axis of the vehicle, which said equalization may possibly entail, are avoided. At the same time, the open-loop or closed-loop control of the level is simplified because the aforesaid differences cannot exert any appreciable influences on the movements of the vehicle body when the level is raised or lowered.
Moreover, with pneumatic suspension systems it is possible to ensure a particularly simple design without a pneumatic pressure accumulator.
The lifting and the lowering movements which take place alternately in terms of the vehicle sides cannot be differentiated by the vehicle occupants given appropriately small lifting or lowering increments, i.e. it is assumed that the vehicle is being raised or lowered on both vehicle sides uniformly. This effect is due to the fact that feeding compressed air into a pneumatic-suspension assembly, or venting a pneumatic-suspension assembly, on one vehicle side leads in each case to corresponding changes in level on both vehicle sides because the tendency of a vehicle to roll is typically limited by at least one stabilizer between vehicle wheels or carriers on different vehicle sides.
Furthermore, in a dual-axle or multi-axle vehicle, the support of the vehicle body on the other axle or axles counteracts a rolling movement. This effect is particularly pronounced if the wheel carriers of the axle assembly are connected to one another via a rigid axle or a coupling which is similar to a rigid axle. This is because in this case a change in level which occurs in the same direction on both sides when a pneumatic-suspension assembly is being adjusted is also effectively supported by virtue of the fact that the pneumatic-suspension assemblies are normally arranged offset to a greater or lesser extent towards the longitudinal centre of the vehicle in the transverse direction of the vehicle in relation to the wheel carriers of the axle assembly.
If, according to one preferred embodiment of the invention, a relatively large offset is provided, the alternating actuation of the pneumatic-suspension assemblies on the two vehicle sides is perceptible to a particularly small degree.
In an advantageous refinement of the invention, the sensors which serve only for sensing the level are assigned signal filters at the output end, which pass on long-term average values of the sensor signals during travelling mode and pass on short-term average values of the sensor signals or unfiltered sensor signals to the closed-loop or open-loop control unit when the vehicle is stationary. This allows for the fact that oscillation movements which occur during travelling mode are normally not associated with an enduring change in level, and changes in level cannot occur, or can only occur owing to leaks in the pneumatic system, and can thus occur comparatively slowly. In contrast, when the vehicle is stationary it is necessary to allow for loading and unloading of the vehicle or for persons getting in or getting out, and correspondingly for severe changes in loading.
Moreover, it is advantageously possible to make provision for level changes which are controlled as a function of displacement to be performed when the vehicle is stationary and for predominantly time-controlled level changes to be performed when the vehicle is travelling. For this purpose, when a change in level is necessary during travelling mode, a pneumatic-suspension assembly can in each case be connected to the compressor or to the outlet for a predefined time period of, for example, five seconds, and then shut off from the compressor and the outlet for a predefined longer time period which is matched to the formation of average values over time by the signal filter. In contrast, when the vehicle is stationary, a change in level occurs which is controlled solely as a function of the sensor signals.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.